Enforce order for elements in unordered data structures

[di]

In #6 @ewdurbin requested some way to enforce the order of the classifiers set and the deprecated_classifiers dict in source.

This is not actually a requirement for this library to work correctly, but would be a nice-to-have for maintainers.

[pradyunsg]

I mean... we can add a pytest test that reads the file, and checks if the set's contents are indeed correctly. It's OK to be slightly fragile at times.

PS: I'm only slightly crazy. :)

Originally posted by @pradyunsg in #6 (comment)